Oxygen can be separated from air or other oxygen containing gases with the use of an oxygen transport membrane. Oxygen transport membranes are fabricated from ceramics that exhibit oxygen ion conductivity at elevated temperatures. An oxygen containing feed stream, for instance, air, is passed along one side of the membrane that is designated as a cathode side. At the cathode side of the membrane, the oxygen ionizes by gaining electrons. The oxygen ions pass through the membrane and emerge from the other side of the membrane, designated as the anode side. At the anode side, the oxygen ions release their excess electrons and combine to form elemental oxygen.
In membranes known as mixed conductors, the electrons from the anode side are transported through the membrane to the cathode side where they serve to ionize the oxygen. In other ceramic materials known as ionic conductors, the membrane will solely conduct oxygen ions. External electrodes are therefore placed at the cathode and anode sides of the membrane and a separate electrical pathway is provided for the conduction of electrons. In dual phase conductors, the electrically conducting pathways are applied as a conductive phase within the membrane itself.
The oxygen separation by oxygen transport membranes requires a driving force of oxygen partial pressure. The oxygen partial pressure at the cathode side of the membrane must be higher than the oxygen partial pressure at the anode side of the membrane for the separation to occur. Typically, the driving force is supplied by compression of the incoming oxygen containing feed. Alternatively, or in addition, as disclosed in U.S. Pat. No. 6,539,719 a vacuum may be drawn at the anode side by a vacuum blower/compressor to decrease the oxygen partial pressure at the anode side. Vacuum blowers are also disclosed for such purposes in U.S. Pat. Nos. 5,516,359, 5,447,555 and 5,935,298. Another manner of reducing the partial pressure at the anode side of the membrane is the use of a purge stream to either consume or sweep away permeated oxygen and thereby lower the partial pressure of oxygen at the anode side. Such use of purge gas is shown in U.S. Pat. No. 5,944,874.
As may be appreciated, the more the partial pressure is reduced at the anode side of the membrane, the less the oxygen containing feed stream need be compressed for a particular output of oxygen. However, the use of vacuum pumps consumes electricity and also adds to the expense of the reactor system.
As will be discussed, the present invention provides a method of separating oxygen from an oxygen containing feed in which the partial pressure of the oxygen is reduced at the anode side of the membrane without the use of a vacuum blower and like devices.